Twitter is refusing to link users to Kickass.to, the second largest torrent index on the Internet. People who attempt to access the site through Twitter get a warning that the site may be unsafe and potentially harmful. Questions to Twitter about the reason for this unusual blockade remain unanswered.

With millions of unique visitors per day KickassTorrents is one of the most used torrent sites, trailing only behind The Pirate Bay.

The site indexes millions of torrents, some of which are then shared through Facebook, Twitter and other social media. For example, author Alex Sayf Cummings posted a link to a torrent of his book Music Piracy And The Remaking Of American Copyright in a tweet recently.

Cummings was obviously inviting people to grab a free copy from the site, but Twitter is not making that easy. Those who click on the link in question are presented with an ominous warning.

“The site you were trying to visit may be unsafe! This link has been flagged as potentially harmful.”

In the grey-on-black fine print in the bottom right corner, Twitter points out that people can proceed to the site at their own risk, but not everybody may be that gutsy.

The big question of course is why Twitter classified KickassTorrents as an unsafe website. TF contacted the company for an explanation, but 24 hours later we have yet to hear back.

On its website Twitter claims to use Google’s safe browsing diagnostic tool, but the Kickass.to domain is not blocked here. Perhaps even stranger, KickassTorrent’s old domain Kat.ph gets the same blocking treatment.

It appears that Twitter has decided on its own that the torrent site is a no-go zone for users. This is an option, as the site explains in one of its help sections.

“Even if Google’s diagnostic report of the URL is clean, we may decide to continue blocking the URL on Twitter as potentially harmful,” Twitter notes.

Whether the fact that KickassTorrent’s links to a lot of copyright infringing content has anything to do with the blockade is unknown. Previously Facebook restricted access to The Pirate Bay for the same reason, but this decision was reversed after a few months.

Thus far, Kickass.to appears to be the only large torrent site that’s blocked by Twitter, but perhaps it’s just the start?